fix: typo in the "from allowed to enroll to unenrolled" transition state

[Agrendalath]

Description

This fixes the typo in the ManualEnrollmentAudit.state_transition.

Testing instructions

1. Go to the Instructor Dashboard -> Membership.
2. Enroll and then unenroll [email protected] in the course.
3. Go to the Django shell and run from common.djangoapps.student.models import ManualEnrollmentAudit; ManualEnrollmentAudit.objects.last().state_transition.endswith('unenrolled'). It should return True.

Deadline

"None"

Other information

Private-ref: TBD